This invention relates to an electrolyte for electrolytic capacitors and more particularly, for an aluminum electrolytic capacitor for use at high voltage.
Current capacitor technology teaches the use of electrolytes composed of straight- and branched-chain dicarboxylic acids in ethylene glycol as replacement for glycol borate electrolytes for high voltage use. The new electrolytes provide longer life and better parameter stability at 105.degree. C. Commercial capacitors containing such electrolytes are available up to 500 V rating. All of these electrolytes contain ethylene glycol as a solvent, and are limited to use at temperatures no lower than -25.degree. C. Below that temperature, the electrolyte resistance becomes so large that the capacitor equivalent series resistance (ESR) is too high for the capacitor to be useful in an electronic circuit.
Electrolytic capacitors are well known for use in a variety of electronic equipment used for data processing, communication, entertainment and power electronics. Generally, these capacitors are made of a capacitor element which is comprised of anode and cathode foils separated by layer(s) of spacer paper. The electrode foils are generally made of aluminum which has undergone etching to increase surface area. The anode foil has undergone an additional formation process to grow an anodic barrier oxide layer that serves as the capacitor dielectric. Short lengths of wire or strips of narrow sheet stock are mechanically bonded to the lengths of anode and cathode foil in the capacitor element; these are used to make electrical connection to the external circuit. The capacitor element is placed into a protective case. Depending upon the capacitor design and size, electrolyte is impregnated into the capacitor element either before or after it is placed in the case. Details of capacitor design and assembly procedures vary with manufacturer. Generally, if wire connectors are used, they pass through an elastomer seal in the top of the case; sheet strip is welded or riveted to terminals embedded in a rigid polymer cover. The cover is then tightly sealed to the case.
Examples of prior art references include U.S. Pat. No. 3,638,077 which teaches an electrolyte capable of being utilized up to a maximum of 400 volts which contains butyrolactone, ethylene glycol, boric acid and tributylamine. Further electrolytes for use in aluminum electrolytic capacitors are taught in U.S. Pat. No. 4,024,442 which teaches an electrolyte composition comprising an acid selected from the group consisting of benzoic acid and toluic acid, ethylene glycol, and at least one organic base which partially neutralizes the acid, such as triethylamine.